Ultrasound monitoring and imaging systems use transducer arrays to create short high frequency acoustic pulses that undergo reflection from surface interfaces at which there are changes in acoustic impedance. The transducers convert reflected energy into electrical signals that are processed to generate two-dimensional or three-dimensional image information descriptive of a subject under study.
There are a number of applications in which large area ‘patch’ type ultrasound transducer arrays can be used, such as cancer screening and continuous non-invasive blood pressure monitoring. Depending on the application, the element count for the large area transducer can range from 10,000 to >1,000,000. Given the large number of transducer elements, each with its own respective signal processing circuitry, significant power, cost, and area penalties exist.
One way to reduce the number of signal processing channels for such a large area array is through the use of a Mosaic Annular Reconfigurable Array. Capacitive Micromachined Ultrasonic Transducers (cMUTs), which are Micro-Electro-Mechanical Systems (MEMS) structures are also alternatives to traditional PZT-based ultrasound transducers.
With respect to ultrasound probe applications, the transducer arrays in ultrasound probe assemblies typically span an area of about 10 cm2. For new medical applications, such as screening for internal bleeding and tumors, much larger arrays, on the order of 1000 cm2, are required. In non-medical applications even larger arrays are desired.
Such large arrays may be formed by tiling a large number of transducer modules, with each transducer module comprising a subarray of transducer cells and an integrated circuit coupled to the subarray. However, performance of a large transducer area is significantly degraded when there are significant gaps as well as variations in spacings between modules.
The Mosaic Array architecture typically groups a number of subelements together along iso-phase lines to form larger transducer elements which are then each connected to a single system channel. In this way, an array that has tens of thousands of active acoustic subelements can be reduced to a much smaller number of system processing channels (e.g. 20-100). This greatly reduces the requirements on the system and makes possible low power and low complexity electronics systems for large area arrays. In order to realize such an array architecture, the switching electronics are typically integrated directly behind the acoustic array. These switching circuits, which are realized using dedicated ASICs, connect directly to each respective subelement and can be programmed to short these elements to one another in a reconfigurable manner. One of the main challenges with such a system is interconnection of the large number of transducers with a respective switching circuit on the adjacent ASICs.
Acoustic transducer cells are typically multi-layered structures comprising piezoelectric or micro-machined transducers configured with electronic circuitry in a probe assembly. The electrical signals are further processed by beam forming circuitry, typically external to the probe assembly, to generate and display images of structures being studied.
For ultrasound probes, it is desirable to contain a portion of the beam forming circuitry integrated with the transducer array, as this can reduce complexities and potentially adverse effects which may result from connecting cables between the transducer probe and an external system that provides signal processing and control functions. For example, with connecting cables extending over distances on the order of several meters significant capacitance effects can arise. Furthermore, signals received from the transducer assembly may be weak, subject to RF interference and may exhibit an undesirably low signal-to-noise (S/N) ratio. To mitigate these effects, front-end circuit cells providing, for example, amplification, pulse generation, and transmit/receive switching, can be integrated with a transducer array.
In the conventional field, sensor/ASIC assemblies are typically not tileable and modularized. Arrays of such assemblies have been built using, for example, flex-based interconnect or wire bonding. Stacked assemblies have been built using interposers and flip-chip bonding of multiple components. However there is an interest in the structure and processing of transducer arrays that alleviate the problems encountered in the existing designs.